In the world of "Northern Electric" the prefix letters "MD" mean "Manufacturing Deviation". The number 663 as best I can tell from other telephones and related equipment, was a number assigned to the device which was manufactured to the particular customer's specifications. It may have been made for a private company, the above ground offices of a mining company or a railway somewhere, who knows. A Bell Canada technician gave me me the information, originally, and I've run across these oddities several times. I've attached a picture of the round base schematic of a Northern desk stand that most collectors would immediately think was an "N50-AL". But if you look at the very bottom of the schematic, it's been re-classified as an "MD-704" Desk Stand. A collector friend of mine who has the set, sent me the picture of the base schematic. I have one or two subscriber sets with the "MD" prefix, and an odd ball 3 digit number assigned to them.
Jeff Lamb
